Colloidal crystals (3D periodic structures formed from monodisperse colloids) have been extensively explored due to their important usefulness in applications such as diffractive optical devices chemical and bio-sensors and high-density magnetic data recording materials. Recently they have attracted renewed interest, mainly because they provide a much simpler, faster and cheaper approach than complex semiconductor nanolithography techniques to create 3D photonic crystals working in the optical wavelength range.
Spontaneously organized colloidal crystals of submicron spheres have provided convenient 3D templates for the construction of macroporous photonic crystals sometimes called “inverted opals.” In this approach, voids between colloidal spheres are infiltrated with a semiconductor material. Subsequent removal of the colloidal-sphere template, by either wet etching or thermal decomposition, leads to the formation of 3D ordered air cavities inside high refractive index materials.
Polymeric replicas of colloidal crystals—macroporous polymers created by the same templating approach—have successfully been demonstrated in varied applications, including use as separation media for macromolecules and DNA separation, biosensors, and “lost-wax” scaffolds for building complex colloids and colloidal crystals, as well as in optical applications. They are also promising candidates for low-k dielectric materials to reduce signal delay and cross-talk in interconnects within integrated circuits.
A variety of methods that use self-assembly can create colloidal crystals with millimeter to centimeter-sized single- or poly-crystalline domains in a time period from days to weeks. Although such methods are favorable for low volume, laboratory-scale production, scaling-up to industrial-scale mass-fabrication seems infeasible due to their tedious fabrication processes and incompatibility with the wafer-scale batch microfabrication techniques widely used by the semiconductor industry. In addition, these methods lead to non-uniform or non-controllable thickness as well as many unwanted structural defects, such as evaporation-induced cracks, which can destroy photonic band gaps and hinder the successful fabrication or development of practical devices.
These problems also impact the fabrication of macroporous polymers, as most fabrication methods for macroporous polymers involve the pre-formation of colloidal crystals as structural scaffolds.